1. Field of the Invention
The present invention generally relates to a method of making a magnet alloy by rapidly cooling and solidifying a molten alloy.
Specifically, the present invention relates to a method of preparing a material alloy for nanocomposite magnets for use in various types of motors, meters, sensors and loudspeakers. More particularly, the present invention relates to a method of making a rapidly solidified alloy for use to produce a nanocomposite magnet in which soft magnetic phases such as iron-based borides and iron and a hard magnetic phase such as an R2Fe14B compound (where R is a rare-earth element) are magnetically coupled together. It should be noted, however, that the rapidly solidified alloy prepared by the method of the present invention can be used effectively not only in nanocomposite magnets but also in bonded magnets (including rubber magnets) and sintered magnets as well.
The present invention further relates to a magnet powder obtained by pulverizing the rapidly solidified alloy and to a magnet body made of the magnet powder.
2. Description of the Related Art
A nanocomposite magnet, having a structure in which a hard magnetic phase such as R2Fe14B and soft magnetic phases such as Fe3B (including Fe3.5B) and α-Fe are magnetically coupled together via exchange interactions, is now under development as an R—Fe—B based magnet. A powder of a nanocomposite magnet is compacted into a predetermined shape with a resin binder, thereby forming an isotropic bonded magnet.
In producing a nanocomposite magnet, a rapidly cooled and solidified alloy, having either an amorphous structure or at least a structure consisting mostly of amorphous phases (which will be referred to herein as a “rapidly solidified alloy”), is often used as a starting material thereof. When subjected to a heat treatment, this rapidly solidified alloy crystallizes and eventually becomes a permanent magnet material having a nanocomposite structure with an average crystal grain size of about 10−9 m to about 10−6 m.
The structure of the heated and crystallized alloy heavily depends on the structure of the rapidly solidified alloy that is yet to be heated and crystallized. For that reason, to obtain a nanocomposite magnet having excellent magnetic properties, it is important how to define the conditions of rapidly cooling and solidifying a molten alloy because those conditions should determine the specific structure (e.g., the percentage of amorphous phases) of the resultant rapidly solidified alloy.
A rapid cooling process to be performed with a machine such as that shown in FIG. 1 (i.e., a melt spinning machine) is known as a conventional method of preparing such a rapidly solidified alloy including a greater volume percentage of amorphous phases. In this process, a molten alloy is ejected out of a nozzle, having an orifice with an inside diameter of about 1 mm or less at the bottom, toward a rotating chill roller, and rapidly cooled and solidified by the roller, thereby obtaining a thin-strip amorphized solidified alloy.
Methods of this type have been researched and reported by universities and organizations that are engaged in the study of magnetic materials. However, a machine for use in those research studies or reports is modeled just for experimental purposes so as to melt several grams to several hundreds of grams of alloy inside of a nozzle and eject it out of the nozzle. That is to say, a machine having such a low processing rate cannot mass-produce a material alloy for a nanocomposite magnet.
Although not designed specially to make a magnet alloy, a machine including multiple nozzles to eject a molten alloy toward a chill roller is described in Japanese Laid-Open Publications No. 2-179803, No. 2-247304, No. 2-247305, No. 2-247306, No. 2-247307, No. 2-247308, No. 2-247309 and No. 2-247310, for example.
In these methods, a molten alloy, which has been melted in a melting crucible, is teemed into a container having ejecting nozzles at the bottom, and then ejected out of the nozzles toward the surface of a rotating roller by applying a predetermined pressure onto the melt in the container (this method will be referred to herein as a “melt spinning process”). By ejecting the melt through the nozzles while applying a pressure thereto in this manner, a stream of the melt (or a melt flow) having a relatively high flow rate can be ejected substantially perpendicularly toward around the top of the rotating roller. The ejected melt forms a puddle (i.e., a melt puddle) on the surface of the chill roller that is rotating at a relatively high velocity (e.g., at a roller surface velocity of about 20 m/s or more). A portion of this puddle, which is in contact with the roller, is rapidly cooled and solidified, thereby forming a thin-strip rapidly solidified alloy.
In the melt spinning process described above, the molten alloy and the rotating roller have just a short contact length. Accordingly, the melt cannot be rapidly cooled and solidified fully on the rotating roller, and the alloy at a high temperature (e.g., about 700° C. to about 900° C.) is cooled and solidified efficiently enough due to its small thickness (typically about 40 μm or less) even after having left the rotating roller and while traveling in the air. In the melt spinning process, the cooling process is carried out in this manner, thereby amorphizing any of various types of alloys.
The applicant of the present application also disclosed a method of producing a nanocomposite magnet by a strip casting process in Japanese Patent No. 3297676 and PCT International Publication WO 02/30595 A1. Furthermore, known strip casting machines and processes using a tundish are disclosed in Japanese Laid-Open Publications No. 11-333549 and No. 2000-79451, for example.
In the melt spinning process, however, the molten alloy is ejected through the nozzle with a small inside diameter, and receives a strong resistance from the nozzle. Thus, it is necessary to apply a sufficiently high pressure to the molten alloy constantly. Such a high pressure is normally applied by adjusting the weight (and the back pressure if necessary) of the melt itself. Accordingly, a rather heavy melt is always reserved to substantially the same level above the nozzle.
Furthermore, the melt is ejected through the nozzle with such a small inside diameter. Accordingly, once a portion of the nozzle has been clogged up with the melt, the melt starts to receive an even higher resistance from the nozzle, thus possibly varying the ejecting rate of the melt.
As a result, in the conventional melt spinning process, if one tries to raise the productivity of the rapidly solidified alloy by increasing the melt feeding rate to about 1.5 kg/min or more, for example, then it becomes hard to control the melt feeding rate at a constant value. Consequently, the rapid cooling rate likely varies considerably which affects the resultant magnetic properties significantly.
Also, in the melt spinning process, a rapidly solidified alloy including a greater volume percentage of amorphous phases is obtained by ejecting a small amount of melt onto a chill roller that rotates at a relatively high velocity (e.g., at a peripheral velocity of about 20 m/s or more). Thus, the resultant thin-strip rapidly solidified alloy typically has a thickness of about 40 μm or less. It is difficult to collect a thin-strip alloy having such a small thickness so as to efficiently increase the tap density thereof by a sufficient amount. Furthermore, powder particles, obtained by pulverizing such a rapidly solidified alloy with a thickness of about 40 μm or less, have a flat shape. Accordingly, those powder particles exhibit a poor flowability or loadability and result in a low magnet powder fill density in a compaction process, thus often decreasing the magnet powder percentage in the resultant bonded magnet.
On the other hand, a strip casting process is also known as another method of preparing a rapidly solidified alloy as described above. In the strip casting process, a molten alloy is supplied from a melting crucible onto a shoot (or tundish) and then brought into contact with a chill roller, thereby making a rapidly solidified alloy.
Hereinafter, the strip caster and strip casting method as disclosed in Japanese Patent No. 3297676 and PCT International Publication WO 02/30595 A1 will be described with reference to FIG. 2.
As shown in FIG. 2, the strip caster includes a melting crucible 11, a shoot (i.e., a guide member) 14, and a chill roller 13. The melting crucible 11 is provided to melt a material alloy and store the molten alloy therein. The shoot 14 receives the molten alloy 12 that has been teemed from the melting crucible 11 and guides the molten alloy 12 to a predetermined location. Then, the molten alloy 12 is teemed from the end of the shoot 14 onto the chill roller 13 and rapidly cooled and solidified by the chill roller 13.
The shoot 14 includes a melt guide surface, which defines a tilt angle β with respect to a horizontal plane, and controls the flow velocity of the melt running down on the guide surface and rectifies the melt flow, thereby feeding the melt onto the chill roller 13 constantly and continuously.
The molten alloy 12, which has come into contact with the outer circumference of the chill roller 13, moves along the circumference of the roller 13 so as to be dragged by the rotating chill roller 13 and cooled in the meantime. Then, the resultant thin-strip rapidly solidified alloy 15 leaves the chill roller 13. In the strip casting process, the angle α, which is defined by a line that connects a point of contact between the molten alloy 12 and the chill roller 13 (i.e., the location of the melt puddle) to the axis of rotation of the chill roller 13 with respect to a vertical plane, is an important parameter. Suppose the angle α is defined to be positive in the direction opposite to the rotational direction of the chill roller 13. In that case, the greater the angle α, the longer the length of contact portion between the molten alloy 12 and the chill roller 13. In a melt spinning process, the chill roller 13 normally has a relatively high rotational peripheral velocity. Accordingly, unless this angle α is defined substantially equal to zero degrees, the molten alloy 12 is easily splashed off by the rotating chill roller 13. For that reason, in a melt spinning process, the angle α is normally approximately zero degrees and the contact portion between the melt and the chill roller is usually relatively short. In contrast, in the strip casting process, the angle α may be relatively large, the contact portion between the molten alloy and the outer circumference of the roller may be relatively long as measured in the roller circumferential direction, and the molten alloy can be cooled almost completely while on the roller.
As described above, the conventional strip casting process uses no ejecting nozzle unlike the melt spinning process but feeds the molten alloy 12 continuously onto the rotating roller 13 by way of the shoot 14. Thus, the strip casting process is effective for mass production and can reduce the manufacturing cost.
In such a strip casting process, however, the molten alloy being fed from the guide member onto the chill roller has a small kinetic momentum. Accordingly, if the chill roller is rotated at a relatively high velocity, then the degree of contact between the molten alloy and the surface of the chill roller will be too low to form the melt puddle constantly on the surface of the chill roller and to obtain a thin-strip rapidly solidified alloy with a uniform thickness. Thus, even if one tries to make a rapidly solidified alloy having a nanocrystalline structure for a nanocomposite magnet by using the conventional strip caster, the resultant thin-strip rapidly solidified alloy will have non-uniform thicknesses and structure. Consequently, it has been difficult to produce an actually usable rapidly solidified alloy constantly by the conventional strip casting process.
On the other hand, when a strip caster including a tundish as disclosed in Japanese Laid-Open Publications No. 11-333549 and No. 2000-79451 is used, the melt flowing on the tundish has a low flow velocity, and the resultant rapid cooling rate tends to be low. Thus, such a machine has been regarded as ineffective for making a rapidly solidified alloy for a nanocomposite magnet. The reason is as follows. Specifically, if the rapid cooling rate is low, an alloy including a lot of crystal structures with a relatively large grain size is formed easily. Also, the crystal grains of soft magnetic phases such as an α-Fe phase likely have such excessive grain sizes so as to significantly deteriorate the magnetic properties in many cases.
For that reason, the strip caster as disclosed in Japanese Laid-Open Publications No. 11-333549 and No. 2000-79451 is often used to make completely crystallized ingots of a metal. A rapidly solidified alloy obtained in this manner is normally used as a material alloy for a sintered magnet including an R2Fe14B phase as its main phase, and cannot be used as a material alloy for a nanocomposite magnet in which hard and soft magnetic phases of very small sizes are distributed uniformly in the same metal structure.